ANCHORAGE, Alaska  A conservation group, alarmed at a decrease in the number of sea otters in southwest Alaska, filed a lawsuit in federal court on Tuesday to try to compel the government to designate critical habitat to help the endangered species recover.

The lawsuit, filed by the Arizona-based Center for Biological Diversity, argues that the U.S. Fish and Wildlife Service missed an Aug. 9 deadline for the designation under the Endangered Species Act. If granted, the designation means that federal agencies must ensure activities in certain areas do not harm the species.

The U.S. Fish and Wildlife Service generally is required to designate critical habitat when a species is listed as endangered or within a year if it can't be done immediately. The sea otter was put on the list in August 2005.

"Sea otters in southwest Alaska are in a grave situation with alarming and ongoing population declines," the lawsuit says. "The absence of critical habitat permits the degradation, modification, and destruction of habitat essential to the Alaska sea otter's survival and recovery."

Douglas Burn, a wildlife biologist with U.S. Fish and Wildlife in Anchorage, said while he can't comment on the lawsuit the agency is not ignoring the issue. He said a team of experts is helping develop a recovery plan for the sea otter and has discussed the role of critical habitat.

The 1,000-mile long Aleutian Island chain once had an estimated 75,000 sea otters -- slightly more than the current statewide total.

Now, there are about 8,700 sea otters in the Aleutians and numbers for the southwestern region, which includes the Aleutians, have dropped by more than half, said Burn. The are an estimated 73,000 sea otters in the entire state now.

The reason behind the sea otter population's collapse is not known, although some attribute it to increased predation by killer whales and climate change that may be reducing available prey.

Miyoko Sakashita, a lawyer with the Center for Biological Diversity, said opening up areas in the Bering Sea and Bristol Bay to oil exploration, as has been proposed, could further devastate the sea otter population.